Method and apparatus for making ceramic molds



G. M. BROWN Aug. 23, 1966 2 Sheets-Sheet 1 Filed Nov. 5, 1962 F/G ZINVENTOR.

GEORGE M, BROWN G- M. BROWN Aug. 23, 1966 METHOD AND APPARATUS FORMAKING CERAMIC MOLDS 2 Sheets-Sheet 2 Filed Nov. 5, 1962 M mi INVENTOR.

GEORGE M BROWN HA5 flTTO/ NEY United States Patent 3,267,527 METHOD ANDAPPARATUS FOR MAKING CERAMIC MOLDS George M. Brown, 7536 JehnnycalreRidge Road, Mentor Township, Lake County, Ohio Filed Nov. 5, 1962, Ser.No. 235,245 3 Claims. (Cl. 2214) This invention relates to investmentmolding and more particularly concerns the formation of ceramic moldsdefining impellers with blades having a spiral configuration.

Heretofore, precision casting of complex bladed impellers for pumps andrelated devices has conventionally been accomplished by the use ofdisposable patterns, such as wax, in the formation of ceramic moldsdefining the bladed casting cavity.

One of the objects of this invention is to provide a new and improvedmethod and apparatus for forming ceramic molds for precision impellershaving screw-type blades.

Another object of this invention is to provide a new and improved methodand apparatus for forming ceramic molds for impellers of the typedescribed by use of a permanent positive pattern.

A further object of this invention is to provide a new and improvedmethod and apparatus for twisting a positive impeller pattern of thetype described out of a ceramic mold.

An additional object of this invention is to provide a new and improvedmethod and apparatus for forming ceramic molds for impellers of the typedescribed which readily lends itself to the demands of economicmanufacture.

Other objects and advantages will become apparent from consideration ofthe following specification taken in conjuction with the accompanyingdrawings in which like characters refer to the same of similar partsthroughout, and in which:

FIG. 1 is a perspective view showing the pattern of an impelleraccording to the invention.

FIG. 2 is an elevational view of an assembled flask, partly in sectionto show an invested pattern according to the invention.

FIG. 3 is a top plan view of a flask according to the invention.

FIG. 4 is a front elevational view of a twist-off mechanism according tothe invention.

FIG. 5 is a side elevational view of the twist-off mechanism of FIG. 4.

FIG. 6 is a fragmentary View, partly in section, showing the cross-slotrail securing arrangement according to the invention.

FIG. 7 is a fragmentary view, partly in section, showing operation ofthe flask assembly and twist-off mechanism according to the invention.

Referring now more particularly to the drawings, FIG. 1 shows a metalpattern 14 which is identical to the impeller to be cast except for theaxial standard 15 at the hase thereof. The impeller pattern has aconical hub with one or more blades or fins 16 radiating therefrom in ahelical or spiral configuration. Plural blades may be of varying lengthbut each must terminate at the base end 17' of the hub and have the sameconstant lead angle to permit their simultaneous withdrawal from theceramic mold. In addition to an identical constant lead angle for allblades, it is essential that the spacing between adjacent blades, or ifone blade, between adjacent turns, be constant. A positive masterpattern of the impeller is usually machined to shape and provided with apolished finish, usually on the order of 20 microinches R.M.S. ofsurface roughness. The pattern is usually cut with a A ice tapered endmill having a 1 degree included angle to compensate for flex of theradially outward portion of each blade so as to prevent radial backdraftwhich would preclude twisting of the pattern from the ceramic mold.

Referring now more particularly to FIGS. 2 and 3, the impeller 14 isshown secured to a cylindrical flask base 19, with the impellersstandard 15 engaged in axial bore 20. Base 19 is further formed with anoff-center bore 21 to receive bolt 22 therethrough for threadedengagement with :a suitably tapped hole in the base of impeller 14 toprevent rotation thereof on the base 19. Base 19 has four equally spacedfeet 24, each projecting from the underside thereof so as to defineopposed slots 25.

Adapted for engagement with flask base 19 is cylindrical flask 27 whichhas booking halves 2S and 29 releasably held by recessed bolts as at 31.is formed with a pair of opposed ears 32 projecting radially andextending along the side of flask 27 for slidable engagement in keyslottracks of a twist-oif mechanism as will hereinafter be seen. Toreleasably secure flask 27 to base 19, flask 27 is formed with opposedbores, as at 37, through each of which a bolt, as at 338, is adapted tobe inserted for threaded engagement with the tapped hole 40 in base 19.A circumferential step 41 is also provided between flask 27 and base 19to maintain axial alignment of the flask assembly. Flask 27 is formedwith a conical interior wall 33 terminating at the upper end thereof ina central sprue opening 34 through which a slurry of self-bondingrefractory mix 42 is adapted to be disposed to invest pattern 14.

Conical wall 33 is shown spaced from the exterior envelop of pattern 14.The configuration of the interior of flask 27 will depend on the methodand apparatus used for investing and casting. It has been found that aspacing of A1 to /2 inch provides suflicient strength to permit theceramic mold to readily withstand the twist-01f operation, whereconventional investments and investing techniques are utilized. Wherethe blade is to extend through to the conical wall 33, it is oftenadvantageous. to loosely wind fiberglass or wire stringers about pattern14 to be invested with the pattern to provide additional strength. And,of course, the pattern 14 and the interior of flask 27 is normallysprayed with a release agent, such as silicone, prior to investing.vDuring or after the setting of the investment, flask securing bolts 38are removed and the flask assembly is placed inthe twist-off mechanismshown in FIGS. 4 and 5.

Referring now more particularly to FIGS. 4 and 5, numeral 44 designatesa rectangular base to which spaced parallel rails 45 and 46 are securedby :bolts 47. Rails 45 and 46 are each formed along the top thereof withinwardly opening channels, as at 48, which are adapted to slidablyengage theears 32 of flask 27.

Spaced from one end of the rails 45 and 46 is a rotatably supportedchuck 49 having four radial keyslot tracks 50 equally spaced thereabout.Tracks 50 are adapted to axially receive feet 24 of flask base 19 sothat upon rotational movement of chuck 49, feet 24 engage with tracks 50to prevent further axial movement of flask base 19.

Rails 45 and 46 are selectively positionable to accommodate differentsize flask assemblies by providing a cross-slot arrangement for each ofthe rail securing bolts 47. As shown in FIG. 6, elongate slot 53 in thefooter 54 of rail 46, and perpendicularly disposed elongate slot 55 inthe base 44, are in confronting registration so as to define a holetherethrough to receive securing bolt 47,

which is held by a clamping nut on the underside of base Flask half 29Chuck 49 is supported for rotational movement by drive shaft 60 that isdriven by reversible motor 61 through gear reducer 62 and pillow block63. Direction of rotation of chuck 49 is determined by a suitablemanually operated motor start switch 64. This arrangement provides forthe chuck being driven in either a clockwise or counterclockwisedirection depending on whether the pattern has a left-handed orright-handed spiral.

Operation of the twist-off mechanism and flask assembly may best beunderstood by reference to FIG. 6. The flask assembly, as shown in FIG.2, is supported on rails 45 and 46 with ears 32, slidably engaged inchannels 48 so as to permit only axial movement of the flask assembly.The flask assembly is adapted to be slid axially into the chuck 49 sothat feet 24 of base 19 are received freely within their correspondingtracks 50 in chuck 49, the arrangement being such that thecircumferential length of each foot 24 is less than the opening to track50 in the face of chuck 49. Upon actuation of switch 64, flask base 19rotates with the chuck 49 at 2 to 3 rpm. forcing pattern 14 to rotatetherewith. Initial rotation of chuck 49 in either direction causestracks 50 to engage or lock-in feet 24 so as to preclude axialseparation of the base 19 and chuck 49. Continued rotation of chuck 49effects a like rotation of flask base 19, whereupon impeller pattern 14unscrews or twists-off from the ceramic mold 42 forcing flask 27 alongwith mold 42 to ride axially away from flask base 19 in channels 48, thearrangement being such that flask 27 and flask base 19 separate on aplane at right angles to their axis. The interior wall of flask 27 maybe formed with one or more inwardly projecting ears to assure that themold 42 will not turn with the flask 27 during the twist-off operation.

The twist-off operation should be accomplished at the time of theinitial set of the investment or before appreciable expansion thereofhas taken place. Initial set of gypsum-type investments having a plasterof Paris binder occurs shortly after the material loses its water gloss.The investment will become warm when the set occurs. As soon as thisheat can be felt, the twist-off operation may be accomplished. Thelength of time after the initial set which may expire before twist-offmust be empirically determined for each type of investment. With conventional gypsum-type investments, twist-off should be accomplished Withinto minutes after set, since expansion after set continues for at leasttwo hours. For investment materials which utilize other binders, such ascolloidal silica sol, expansion after setting is much less pronounced.For results, twist-off should be accomplished after setting of theinvestment but before appreciable expansion has taken place, which formost impeller configurations is on the order of .001 inchper inch.

It is to be noted that in the production of molds for impellers having aplurality of blades, the blades of the positive pattern will havesignificant, although slight, variations in blade lead, thickness andspacing due to limitations in the conventional machining methoddescribed. These variations tend to develop stresses in the mold duringthe twist-off operation. The self-guiding twist-01f method hereindescribed inherently provides for balancing or distribution of thesestresses so as to minimize the incidence of mold fracture.

From the foregoing it should be apparent that I have provided a new andimproved investment molding method and apparatus which is well adaptedto fulfill the aforestated objects of the invention. Moreover, whereasthe invention has been described in particularity with respect to apreferred embodiment which gives satisfactory results, it will beunderstood by those skilled in the art to which the invention mostnearly appertains that other embodiments and modifications thereof maybe resorted to without departing from the spirit or scope of theinvention as defined by the appended claims.

I claim:

1. A method of forming a ceramic mold for an impeller having a conicalhub with a plurality of equally spaced screw-type blades of constantlead radiating therefrom, including the steps of; forming a positivepattern of the impeller so that the spacing between adjacent turns isconstant, coaxially securing the positive pattern to a cylindrical flaskbase for rotation therewith along its axis, securing a cylindrical flaskto said flask base so as to define an investment cavity about saidpattern, investing the pattern with a slurry of self-bonding ceramicrefractory mix so as to form a ceramic mold within the flask, aftersetting of the slurry but before appreciable expansion thereof twistingthe flask base so as to unscrew the impeller pattern from the ceramicmold which is held in the flask, with the thread formed by the patternin the ceramic mold functioning as the sole pitch guide for movement ofthe pattern whereby the stresses on the mold developed during suchtwisting are equally distributed.

2. In combination, a twist-off mechanism and a flask assembly; saidtwist-off mechanism including a base, spaced parallel rails securedthereto having confronting channels, a rotatable chuck supported fromsaid base spaced from one end of said rails, means for selectivelydriving saidchuck, said chuck being formed with oppositely extendingradial tracks; said flask assembly including a flask and flask base,said flask base having an impeller pattern secured thereto, said flaskassembly adapted to be filled with a self-bonding refractory ceramic mixso as to form a mold about said pattern, said flask having opposed earsadapted to be slidably received within said confronting channels, saidflask base having feet adapted to be received within the radial tracksof said chuck so as to engage said flask base therewith, the arrangementbeing such that upon rotation of said chuck with the flask base engagedthereto, the mold thread formed by the pattern functions as a threadguide causing the flask and mold therewith to move in a direction awayfrom said flask base as guided by said flask ears sliding in saidconfronting channels.

3. In combination, a cylindrical flask assembly adapted for separationinto a flask and flask base at right angles to its axis, said flask basehaving secured thereto a pattern having a plurality of equally spacedscrew-like blades, said flask assembly adapted to be filled with aself-bonding refractory ceramic mix so as to form a mold about saidpattern, a first means for restraining said flask from rotationalmovement while permitting axial movement, a second means for restrainingsaid flask base from axial movement while permitting rotationalmovement, and a third means for selectively rotating said second meansso as to unscrew said pattern from said mold, the arrangement being suchthat the mold thread formed by the pattern functions as the sole pitchguide for separation of said flask assembly.

References Cited by the Examiner UNITED STATES PATENTS 133,592 12/1872More 22-14 914,474 3/ 1909 Barnes 22-14 931,760 8/1909 Hattersley 22-141,544,060 6/1925 Dimick 22-14 1,856,166 5/1932 Nordfelt 22-14 2,509,6925/1950 Miller 22-199 X 3,072,979 1/1963 Abbott 22-18 3,162,592 3/1964Taccone 22-14 FOREIGN PATENTS 3,732 of 2/ 1869 Great Britain.

J. SPENCER OVERHOLSER, Primary Examiner.

MARCUS U. LYONS, MICHAEL V. BRINDISI,

Examiners.

1. A METHOD OF FORMING A CERAMIC MOLD FOR AN IMPELLER HAVING A CONICAL HUB WITH A PLURALITY OF EQUALITY SPACED SCREW-TYPE BLADES OF CONSTANT LEAD RADIATING THEREFROM, INCLUDING THE STEPS OF; FORMING A POSITIVE PATTERN OF THE IMPELLER SO THAT THE SPACING BETWEEN ADJACENT TURNS IS CONSTANT, COAXIALLY SECURING THE POSITIVE PATTERN TO A CYLINDRICAL FLASK BASE FOR ROTATION THEREWITH ALONG ITS AXIS, SECURING A CYLINDRICAL FLASK TO SAID FLASK BASE SO AS TO DEFINE AN INVESTMENT CAVITY ABOUT SAID PATTERN, INVESTING THE PATTERN WITH A SLURRY OF SELF-BONDING CERAMIC REFRACTORY MIX SO AS TO FORM A CERAMIC MOLD WITHIN THE FLASK, AFTER SETTING OF THE SLURRY BUT BEFORE APPRECIABLE EXPANSION THEREOF TWISTING THE FLASK BASE SO AS TO UNSCREW THE IMPELLER PATTERN FROM THE CERAMIC MOLD WHICH IS HELD IN THE FLASK, WITH THE THREAD FORMED BY THE PATTERN IN THE CERAMIC MOLD FUNCTIONING AS THE SOLE PITCH GUIDE FOR MOVEMENT OF THE PATTERN WHEREBY THE STRESSES ON THE MOLD DEVELOPED DURING SUCH TWISTING ARE EQUALLY DISTRIBUTED. 